The present invention relates to a thermal head, and more particularly to a thermal head suitable for use in a printer for recording a high-density image such as a dot-matrix Chinese character printer.
In a dot-matrix Chinese character printer utilizing thermal recording techniques, a thermal head in which a plurality of heating elements and wiring conductors connected thereto are arranged on a substrate, is pressed against a thermal recording medium to heat the recording medium by the heating elements which are supplied with electric power from a drive circuit through the wiring conductors, and is moved in a direction intersecting with a direction, along which the heating elements are juxtaposed.
FIG. 1 shows the arrangement of heating elements and wiring conductors in a typical thermal head of the above kind, and the forms of the heating elements and wiring conductors. Referring to FIG. 1, heating elements 1a to 1g are disposed along a B-direction (hereinafter referred to as "vertical direction") perpendicular to an A-direction (hereinafter referred to as "horizontal direction") which is parallel to the moving direction of the thermal head. In the thermal head of FIG. 1, 24 heating elements are provided along a vertical direction so that relations W.gtoreq.P and H&lt;P are satisfied and P is about 140 .mu.m, where W and P indicate the dimensions of each heating element in horizontal and vertical directions, respectively, and P indicates an interval at which the heating elements are arranged in the vertical direction. Wiring conductors 2a to 2g and 3a to 3g for supplying electric power to the heating elements 1a to 1g are extended therefrom in horizontal directions. End portions of the conductors 2a to 2g are connected to a common electrode 2h, which is connected to a drive circuit (not shown) disposed in a lower part of the thermal head. While, the conductors 3a to 3g extended from the heating elements 1a to 1g in horizontal directions are bent so as to be extended in vertical directions, and then connected to the drive circuit.
In such a thermal head, it is desirable that one dot recorded by one of the heating elements 1a to 1g has the form of a square. That is, it is desirable that the dimension w of one dot in a horizontal direction and the dimension h of the same dot in a vertical direction are both equal to a value d. Further, it is desirable that a vertical line formed by combining a plurality of dots each having the above dimensions has a width equal to the value d and a horizontal line formed by the combination of such dots has a width equal to the value d. Furthermore, it is desirable to form such lines in a very short time by moving the thermal head in a horizontal direction at high speed. Moreover, it is desirable to be able to record dots or lines satisfying a relation w=h=P, at high speed.
However, when the thermal head of FIG. 1 is moved in the horizontal direction to perform a recording operation, the heating elements kept at an elevated temperature heat a thermal recording medium while sliding thereon. Accordingly, the dimension w of a recorded dot in a horizontal direction becomes larger than the dimension W of each heating element in the horizontal direction (for instance, the dimension w is about 200 .mu.m). The dimension w can be reduced by shortening a time during which the heating element is kept at a high temperature enough to perform a recording operation for the thermal recording medium. However, owing to the heating and cooling characteristics of each heating element, it is impossible to make the dimension w equal to the dimension W. The dimension w of a recorded dot in a horizontal dimension can be made small by reducing the dimension W of each heating element in a horizontal direction. In this case, however, the electric resistance of each heating element is reduced. Accordingly, in order to elevate the temperature of each heating element in a short time, it is required to supply a large current to each heating element, and thus power loss in the wiring conductors and drive circuit becomes large. The resistance of each heating element can be increased by making large the resistivity or thickness thereof. In order to make large the resistivity of the heating element, it is required to make the heating elements of limited kinds of materials. Further, when the thickness of the heating element is increased, characteristics of the heating element vary widely, on the basis of manufacturing errors. Accordingly, when the heating elements are made of a resistance material now available, and are formed so as to make the above-mentioned power loss as small as possible, the dimension W of each heating element in a horizontal direction is larger than the dimension H thereof in a vertical direction, and is not smaller than the interval P (that is, W&gt;H and W.gtoreq.P).
While, in order to electrically insulate adjacent heating elements from each other, it is required to provide a gap (equal to, for instance, 15 .mu.m) between adjacent heating elements. Thus, the dimension H is smaller than the internal P. In high-speed recording, it is very difficult to heat that region on the thermal recording medium which is ajacent, in a vertical direction, to an area pressed by the heating element, through heat conduction. Accordingly, the dimension h of a recorded dot in a vertical direction will be indicated by a relation h.div.H&lt;P.
Further, in order to make large the dimension h of a recorded dot in a vertical direction so that a continuous, vertical line can be formed of plural dots, it is required to pass a large current through each heating element, and therefore the dimension w of the recorded dot in a horizontal direction further increases.
As mentioned above, according to the conventional thermal head, a figure or character having a thick vertical line and a thin horizontal line is recorded. The width of the vertical line increases and the width of the horizontal line decreases, as the recording speed becomes higher.
In order to solve the problem resulting from the relation H&lt;P, a thermal head shown in FIG. 2 has been proposed in which heating elements 4a to 4g are arranged in two columns so as to form a zigzag, thereby obtaining a relation H&gt;P, and heating elements in a first column are made different in current flowing time from heating elements in a second column so that one continuous vertical line can be recorded by the heating elements in the first and second columns. Although the relation H&gt;P is achieved by the arrangement shown in FIG. 2, the problem with respect to a decrease in resistance of each heating element caused by reducing the dimension W cannot be solved by this arrangement, and therefore the relation W&gt;H remains as it is. Further, in the arrangement of FIG. 2, the heating elements in the first column and those in the second column are alternately supplied with a current so as to record dots at the same positions on recording paper, and thus a distance between the first and second columns is made equal to (N+1/2)P, where N is a positive integer.
Further, a common wiring conductor 2h' is disposed between the first and second columns, and wiring conductors 3a' to 3g' starting from heating elements 4a to 4g are first parallel to a horizontal direction and then bent downward to be connected to a drive circuit. Accordingly, the thermal head of FIG. 2 has a large width in a horizontal direction. Thus, a high pressure is required to sufficiently press the thermal head against the thermal recording medium. The use of such a high pressure will bring about other problems.